1. Field of the Invention
The present invention relates to a projection lens for use in combination with a cathode ray tube, and more particularly to an improved projection lens capable of substantially and advantageously implementing a distinct picture quality by compensating for chromatic aberrations and residual aberrations.
2. Description of the Conventional Art
Conventionally, a preferred form of projection lenses for wide screen television is disclosed in U.S. Pat. Nos. 4,300,817, 4,348,081, and 4,526,442.
In these previous patents, the lens units have been referred to as groups which perform specified optical functions. Here, the term "lens unit" refers to one or more lens elements or lens components which provide a defined optical function or functions in the design of the overall lens.
It is well known that a specified or defined optical function(s) of a lens or group in an overall lens may be accomplished by using one or more elements or components depending upon the correction or function desired. A decision as to whether one or more elements are used as a lens unit in an overall lens design may be based on various considerations, including but not limited to the ultimate performance of the lens, the ultimate costs of the lens, acceptable size of the lens, etc.
Accordingly, in the following specification and appended claims, the terms "lens unit" refers to one or more lens elements or lens components which provide a defined optical function or functions in the design of the overall lens.
The lens disclosed in the aforementioned patents generally comprises three lens units: from the image end, a first lens unit having at least one aspheric surface, which serves as an aberration corrector; a second lens unit including a biconvex element which supplies all or substantially all of the positive power of the lens; and a third lens unit having a concave surface towards the image end of the lens, serving as a field flattener, and essentially correcting for Petzval curvature of the lens.
The lenses, as disclosed, are designed for use with a surface of a cathode ray tube (CRT). The lenses of U.S. Pat. No. 4,300,817, utilizing a single biconvex element in the second lens unit, all have an equivalent focal length (EFL) of one hundred twenty-seven millimeters or greater, while the lenses of U.S. Pat. No. 4,348,081, which utilize a two-element second lens unit, including the biconvex element, may have an EFL reduced to eighty-five millimeters as designed for direct projection for a five inch diagonal CRT. The lenses described in U.S. Pat. No. 4,526,442 are designed to have a fold in the optical axis between the first and second lens units and have been designed so that the EFL is as short as one hundred twenty-six millimeters. These EFLs are provided for CRT screens having a viewing surface with an approximate five inch diagonal.
projection TV sets are rather bulky and have required high volume cabinet. One manner of reducing the cabinet size is to decrease the EFL of the projection lenses. This, of course, increases the field angle of the lens.
The EFL of the lens is a function of the total conjugate distance between the CRT and the display screen.
This is shown by the relationship: EQU OL=EFL(1+1/M)+EFL(1+M),
where OL is the overall conjugate distance of the system from object to image, PA1 EFL (1+M) is the distance from the image to the first principal point of the lens, PA1 EFL (1+1/M) is the distance from the object to the second principal point of the lens and PA1 M is the magnification of the system expressed as the ratio of object height to image height.
Therefore, in order to decrease the total distance between the CRT and the screen, it is necessary to reduce the EFL, or alternatively stated, increase the field angle of the lens.
An effect of increasing the angular coverage of the lens as a result of decreasing the EFL is that the aberrations become more difficult to correct.
A further consideration is introduced wherein a spacing is provided between the phosphor screen of the CRT and the third lens unit of the projection lens. This spacing may be required for the inclusion of a liquid cooling and/or optical coupling material and a housing necessary to enclose the coolant against the face of the CRT. This additional spacing between the face of the CRT and the third lens unit causes the third lens unit to contribute more negative power, which must be compensated by increased power in the positive second lens unit. In some cases, the phosphor surface of the CRT is curved concave to increase the corner brightness. This permits a power reduction in the third group inasmuch as the power requirement for correction of field curvature is reduced.
A single biconvex element of the second lens unit, as shown in the aforementioned patents, does not provide the lens designer adequate degrees of freedom to correct for the chromatic aberration. By dividing the optical power of the second lens unit, as disclosed in U.S. Pat. No. 4,348,081, a better control of aberrations can be obtained for a shorter EFL. However, merely splitting the optical power of the second lens unit into two elements to obtain additional degrees of optical design freedom does not provide acceptable contrast and resolution where the angular coverage of the projection lenses is required to be in excess of twenty-seven degrees.
Since the advent of lenses, as shown in U.S. Pat. No. 4,300,817, which made large screen home projection television sets feasible, there have been continuing efforts to design projection lenses with wider field angles which are more compact and easier to manufacture at the greatest economy. This, of course, is an effort to reduce the cost of the lens and to reduce the depth of the housing of the television system while maintaining or increasing the size of the viewing screen.
Projection lenses of the overall type described hereinabove have been designed with decreased EFLs by designing a more complex second lens unit split into more than one lens element, as exemplified in the lenses disclosed in U.S. Pat. Nos. 4,697,892 and 4,707,684.
These designs are currently used on many wide screen projection television sets and may have an equivalent focal length as low as eighty millimeters. It will be understood that the EFL could be greater if there is a fold in the optical axis between the first and second lens units.
This approach works very well and leads to high quality optical performance of the lens. However, it requires large diameter positive elements in the second lens unit to accommodate the diverging bundle of light (as traced from the long conjugate). This construction also requires a lens of relatively long front vertex distance (FVD), largely due to a long space between the first negative element and the following power unit, which is necessary to achieve an appropriate correction of field curvature and astigmatism. The front vertex distance is the distance from the image side of the first lens unit to the face place of the CRT.
The related co-pending application discloses a lens of the type which consists of a first lens unit comprising a single element with two aspheric surfaces, and an overall positive meniscus shape preferably convex to the image end, a second lens unit having a positive element, and a third lens unit having a storingly negative surface concave to the image end. The first lens element is of positive optical power at the optical axis of the lens, but due to the aspheric power of the surfaces, the positive optical power decreases with distance from the optical axis and may become strongly negative at or closely adjacent the clear aperture of the first lens element, as hereinafter explained.
The strong negative power of the third lens unit contributes to correction of the Petzval sum of other lens elements. The strongly concave surface may be made aspheric to also correct for residual astigmatism and field curvature of the lens. The second lens element provides the majority of the positive power of the lens and some correction for astigmatism. The first lens element must then correct the aperture dependent aberrations, particularly, spherical aberration and coma aberration. Lenses as described in the related application are very compact, having the first and second lens units spaced more closely than heretofore. Lenses as described may have a field angle as great as 73.degree. while comprising only three elements.
In lenses of the type described in U.S. Pat. No. 4,300,817, all the elements are made out of acrylic because of simplicity of manufacturing aspherical surface on plastic. However, the refractive index of acrylic varies significantly with temperature. This leads to a change in focal lengths of the acrylic lens elements, which in turn, can lead to defocus or lack of sharp focus of the overall lens.
One way of compensating for focus shift with temperature is to design a lens mount and lens barrel using a bi-metallic plate or other means that will shift the position of the lens relative to the CRT as a function of temperature in such a way that the focus of the lens will remain in a constant position. Alteratively, the second or power lens unit may be formed of glass, as disclosed in U.S. Pat. No. 4,348,081, which does not exhibit substantial change in index of refraction with temperature. However, this restricts the lens design in that it is very expensive to define an aspheric surface on glass. A further solution is to design a hybrid lens using a glass power unit with an additional acrylic corrector with one or more aspheric surfaces adjacent to the power unit. However, this does not necessarily provide a lens with a wide field angle and decreased length.
To reduce the cost of manufacturing projection lenses, it is desirable to decrease the size of the elements. In the present invention, a positive first lens unit is utilized to reduce the diameter of the other elements of the lens system. This is achieved through the use of a positive first lens unit preferably in the overall form of a meniscus which converges the rays toward the strongly positive second lens unit (as traced from the long conjugate side). The spacing between the first lens unit and the second lens unit is thereby reduced, which results in a reduction in the front vertex distance of the lens.
To resolve such problems, a conventional lens is well disclosed in U.S. Pat. No. 4,776,681.
Referring to FIG. 1, the conventional projection lens will now be explained.
As shown therein, the conventional projection lenses of the overall type have been designed with a plastic non-aspheric lens, and as shown in FIGS. 2A to 2C, the chromatic aberrations are not corrected.
The conventional projection lenses cannot perform optimum results because the lenses are designed in e-line (546.0 nm) which is a central wave of a green CRT and then the same lenses are used in the red and blue CRTs.
A central wavelength of the blue CRT (450 nm) and a central wavelength of the red CRT (612 nm) are so deviated from the e-line (546.0 nm), which is a central wavelength of a green CRT, so that the chromatic aberrations occur, as shown in FIG. 2A.
FIGS. 3A to 3C show spectrum characteristics which are obtained from the green, blue, and red CRTs.
Since all the CRTs have a part of different wavelength band width, respectively, the chromatic aberrations occur too.
The green lens assembly has the most effect on the display quality of a television. However, the spectrum of the green CRT as shown in FIG. 3A contains the blue and red lights besides the central wavelength, so that the compensation for the chromatic aberrations is required therein.
Generally, the optical system in which the compensation of the chromatic aberrations is not corrected, degrades the display quality and the contrast so that the compensation for the chromatic aberration is required in the high definition television and the high definition video projector.
In these drawings, the lens units are identified by the reference G followed by successive arabic numerals except that a corrector lens unit is designated by the reference CR; lens elements are identified by the reference L followed by successive arabic numerals from the image to the object end. Surfaces of the lens element are identified by the reference S followed by successive arabic numerals from the image to the object end. The reference Si denotes the screen of a cathode ray tube.
The first lens unit G1 comprises an element of positive power and an overall positive shaped meniscus and has at least one aspheric surface defined by the equation: ##EQU1## where X is the surface sag at a semi-aperture distance y from the optical axis A of the lens, C is the curvature of a lens surface at the optical axis A equal to the reciprocal of the radius at the optical axis, K is a conic constant, and D, E, F, and G are aspheric coefficients.
The second lens unit G2 is biconvex and preferably consists of a single biconvex element having spherical surfaces, and is of a glass material to minimize the variation of refractive index with temperature.
The lens unit G3 acts as a field flattener, that is, it corrects Petzval curvature of the first and second lens units. The concave image side surface of the third lens unit G3 may be spherical or aspheric, as hereinafter set forth. As disclosed in U.S. Pat. No. 4,685,774, the spacing D.sub.12 between the first element of lens unit G1 and lens unit G2 is important in aiding in the correction of field curvature. The spacing D.sub.12 between the first and second lens units should be: EQU 0.10&lt;.vertline.D.sub.12 /F.sub.3 .vertline.&lt;0.48
where F.sub.3 is the equivalent focal length of the third lens unit.
If .vertline.D.sub.12 /F.sub.3 .vertline. goes below 0.10, the field curvature becomes over-corrected and the image quality becomes unacceptable. If .vertline.D.sub.12 /F.sub.3 .vertline. exceeds 0.48, the field curvature is under-corrected and the image quality is again not acceptable.
As one attempt to increase the field angle of the lens, more astigmatism is introduced. This may be corrected at the expense of correction of spherical aberration in the second lens unit G2.
The lens unit G1 corrects the spherical aberration introduced by the lens unit G2 as well as coma and some other off-axis aberrations.
This is achieved by providing element L1 with two aspheric surfaces S1 and S2, which define element L1 as having positive optical power at the optical axis which decreases with distance from the optical axis and may change to negative optical power, which becomes very strong at the limit of the clear aperture.
There is set forth the aspheric optical power K.sub.CA /K.sub.A of the first lens element L1 of each prescription at or adjacent the clear aperture from the optical axis to the limit of the clear aperture or just adjacent the limit of the clear aperture. In each case the power at the optical axis is positive and decreases with height y and may become negative and then increase in negative power to an absolute value at least two and one-half times the optical power at the axis with one exception. These relationships exemplify the change in optical power of the lens element L1 from the optical axis to the limit of the clear aperture of the lens.
The optical power K.sub.y of a lens is calculated from the equation: EQU K.sub.y =(n-1) (C.sub.1y -C.sub.2y)
where n is the index of refraction of lens element L1, C.sub.1y is the curvature of the first lens surface at a height y from the optical axis A, and C.sub.2y is the curvature of the second lens surface at the height y from the optical axis.